Certain digital transmission fiber systems require that the system be fully synchronous. In some instances where there is a source of digital information to be connected to the system from a remote station, a method is employed that sends a reference clocking signal to the remote station for the purpose of receiving synchronized data back from the remote station.
One of the problems experienced by conventional systems with this arrangement is that the delay path between the sending clock of the main unit out to the remote location and receiving data back at the main unit is long and dynamically changing due to changes in temperature which effects the equivalent and physical length of the fiber communication path. At present serial bit rates, there can be as much as several bits of variable delay associated with temperature induced changes in the length of fiber.
The standard method of correcting this problem involves the use of a first--in--first out (FIFO) apparatus in the return path so that associated information is maintained in the properly assigned bit locations for insertion into the digital transmission system.
However, this conventional method and apparatus is complex, requires many electronic components and is relatively expensive to employ.
It has been found that simultaneous bidirectional transmission of optical signals may be a desired technique leading to a better functional and cost based system for this application. However, such diplexing techniques of equal wavelength digital bit streams require the use of optical couplers to separate the optical signal information bits. Optical couplers provide a certain degree of optical isolation but reflections of standard non-return to zero (NRZ) line coded signals at the fiber interface will substantially degrade the detected signal to noise ratio.
Thus, in conventional NRZ transmission systems, the spectrum extends from a low frequency determined by the maximum run length of ones or zeros to a maximum frequency approaching the bit rate of the data stream. The low frequency components from the high speed data stream will degrade the signal to noise ratio of the low speed data stream and, conversely, the spectrum of the low speed data stream will contribute noise to the high speed data stream. The result will be errors in the data output. Accordingly, it would appear that use of optical couplers in a diplex mode, single fiber medium would have technical restrictions in the application to remote synchronous digital optical fiber systems.